Polycystic kidney disease

Polycystic kidney disease, or PKD, is a genetic disease in which the kidneys become filled with hundreds of cysts, or fluid-filled sacs, causing them to be larger than normal and to quit functioning over time.

These cysts develop in the outer layer—the cortex, as well as the inner layer—the medulla—of both kidneys.

These cysts, which are lined with renal tubular epithelium, fill up with fluid and get larger and larger over time, making the kidneys much larger than normal.

The blood vessels that feed neighboring healthy nephrons can get compressed by growing cysts, which literally starves them of oxygen.

Poorly perfused kidneys respond by activating the renin-angiotensin-aldosterone system, which facilitates fluid retention and leads to hypertension.

Also, expanding cysts can compress the collecting system, causing urinary stasis, and in some cases this can lead to kidney stones.

Additionally, destruction of the normal renal architecture can cause symptoms like flank pain and hematuria, or blood in the urine.

Over time, as enough nephrons are affected, it leads to renal insufficiency and eventually renal failure.

Now the first type of PKD is autosomal dominant PKD or ADPKD, which used to be called adult PKD, since symptoms usually manifest in adulthood.

The first gene responsible for ADPKD is PKD1, which when mutated causes the more severe and earlier onset variety, and PKD2, which when mutated causes less severe disease and is also later in onset. PKD1 and PKD2 code for the polycystin 1 and polycystin 2 proteins, respectively, which are components of the primary cilium.

Now, the primary cilium is an appendage that sticks out from most cells in the body and receives developmentally important signals.

More specifically, in the nephron, as the urinary filtrate flows by and cause it to bend, polycystin 1 and polycystin 2 respond by allowing calcium influx, which activates pathways in the cell that inhibit cell proliferation.

If either component is absent, that signal to inhibit growth isn’t received, and so cells proliferate abnormally and start to express proteins that cause water to be transported into the lumen of the cyst, which makes them get larger and larger, compressing the surrounding tissue more and more, and this is how the cysts develop and grow.

As expected for a dominant disease, a person who develops ADPKD would have inherited a single, heterozygous mutation in PKD1 or PKD2. This leaves one functional copy of the gene in every cell, and this turns out to actually produce enough polycystin 1 or polycystin 2 to prevent cyst formation.

So how do cysts occur then? Well it turns out that a random mutation in the remaining good copy of the gene is almost guaranteed to happen in some of the tubular cells as the kidney develops. This ‘second hit’ causes polycystin 1 or 2 to be absent and is what impairs normal signalling through the cilium and leads to cyst formation.

So on the level of the person as a whole, ADPKD shows a pattern of dominant inheritance, but on the cellular level it’s technically a recessive trait.

Polycystins are important in the kidney, but are developmentally important in other places of the body, too.